docs(docs): add frontmatter to all sprint briefings

Standardized YAML frontmatter on all 115 sprint briefing files across
sprints 1-26 with title, description, type, status, sprint number, and
team fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/sprints/sprint-2/server.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 2 — Server Briefing"
+description: "Shadowcasting algorithm, vision cone implementation, ObserverSnapshot v2 schema"
+type: sprint
+status: archived
+sprint: 2
+team: "server"
+---
+
 # Sprint 2: See — Server Tasks
 
 **Goal:** Fog of perception working through the bridge — server computes LOS visibility, client renders fog.